The disclosed invention classifies materials by utilizing the tendency of penetrating electromagnetic radiation to pass through differing materials with differing levels of attenuation within the materials according to their chemical properties. The invention provides for separation of the differing materials from each other according to the amount of radiation passing through them. More specifically, penetrating electromagnetic radiation is used to simultaneously scan multiple material items as they pass through a region of radiation. Analysis of the measured radiation passed through differing portions of the body of each item is used to classify each item and activate means for separating from each other items which have differing chemical properties.
It is well known that for materials having similar thicknesses, those materials comprised of elements having a lesser atomic number generally allow a greater degree of penetrating electromagnetic radiation to pass through them than do those materials comprised of elements having a greater atomic number. Additionally, it is also well known that for materials having similar chemical properties, those materials of lesser thickness generally allow a greater degree of penetrating electromagnetic radiation to pass through them than do those materials of greater thickness. Therefore materials of differing chemical properties can be selected according to the amount of penetrating electromagnetic radiation passing through them, if differences in thicknesses of the materials have relatively less effect on the transmission of penetrating electromagnetic radiation through them than do differences in chemistry.
In the recycling of waste or secondary materials it is very useful to be able to separate mixtures of materials into usable fractions, each having similar chemical properties. For instance it is useful to separate plastic materials from glass materials, to separate metals from nonmetals, to separate differing plastics from each other, and to separate dense materials from less dense materials. There are many other such useful separations practiced in industry using many different methods which are too numerous to enumerate herein.
It has been found that in separating mixtures of materials for recycling, the disclosed invention is very effective at distinguishing and separating items of differing chemical composition. Mixtures containing metals, plastics, textiles, paper, and/or other such waste materials can be separated, since penetrating electromagnetic radiation typically passes through the items of different materials to differing degrees. Such mixtures occur frequently in the municipal solid waste recycling industry and in the secondary materials recycling industries. An example is the separation of aluminum beverage cans from mixtures containing such cans and plastic containers. Such mixtures are commonplace in curbside recycling programs. Another example is the separation of chlorinated plastics (a source of corrosive gasses when burned) from a municipal solid waste mixture to provide a less polluting fuel for municipal waste incineration.
It has also been found that the invention is useful for separating chlorinated plastics from mixtures containing nonchlorinated plastics, since it has been found that chlorinated plastics typically allow less transmission of penetrating electromagnetic radiation than do nonchlorinated plastics. Such separation renders each of these plastics more valuable for recycling. Such mixtures of plastics are commonplace in municipal waste recycling programs. Until now such separations have been performed using methods which are cumbersome and slow, thereby limiting their usefulness. For instance in the United States, the manufacturers of plastic containers for consumables have recently begun molding a numerical identification code into the base of the containers. The code indicates chemical composition, such as polyolefins, polyesters, or vinyls (polychlorinated plastics). Using these codes, the, plastics can be manually hand-sorted from each other. However, this method is slow, labor intensive, and expensive and has not found widespread use for these reasons.
There exist three known processes for automated separation of chlorinated plastics from mixtures of plastics according to their response to electromagnetic radiation. One of these processes is disclosed in European patent application No. 88107970.1 of Giovanni, filed May 18, 1988, and published on Nov. 23, 1988. Another process is disclosed in U.S. Pat. No. 4,884,386, issued to Gulmini Carlo on Dec. 5, 1989. The third process is known as the Rutgers process.
Each process requires that items in the mixture be placed singly into a radiation chamber, following which placement measurements are made to classify the plastic item according to its response to an electromagnetic radiation beam. Subsequently the plastic item is directed to a destination according to its chemical composition. After this sequence is completed, another plastic item is fed into the radiation region and the sequence is repeated. This requirement for operation with single items necessitates elaborate equipment for singly selecting items from the mixture and placing them one at a time into these separators. Furthermore, since the plastics are required to be singly classified one after another, the methods are limited in throughput because of the finite time required to execute the sequence for each item.
Typical plastic containers for consumables are manufactured with thicker walls at the neck and base than in their central portions. Such plastic containers, when flattened for storage or shipping reasons during recycling, typically contain folds incurred during the flattening process. Necks, caps, bases and folds give rise to significant variations in total material thickness presented to a penetrating electromagnetic radiation beam. It has been found by the inventors that utilizing measures of radiation transmission through the neck, cap, base, or a folded region of a plastic container can give inaccurate results in attempting to classify the chemical composition of the container due to these variations in total material thickness.